The drive to improve engine combustion efficiency and reducing emissions has resulted in a significant increase of the Turbine Entry Temperatures (TET) within gas-turbine engines since 1940s. Presently, the high-pressure turbine blades and vanes exposed to hot gases in commercial aeronautical engines typically experience metal surface temperatures of about 1000° C., with short-term peaks as high as 1100° C. In this service environment, significant advances in high temperature capabilities have been achieved through the development of iron, nickel and cobalt-base superalloys and the use of oxidation-resistant environmental coatings capable of protecting the superalloys from oxidation, hot corrosion, etc.
For instance, the surfaces of the articles to be protected can be protected with an aluminum-containing protective coating whose surface oxidizes to form an aluminum oxide scale that inhibits further oxidation. The protective coating therefore is sufficiently rich in Al to promote thermal growth of this oxide scale. The scale is also referred to as a thermally grown oxide (TGO). Optionally, a ceramic topcoat is further applied over the aluminum-containing protective layer to help provide a thermal barrier that extends service life. In addition to imparting oxidation resistance, the TGO helps bond the ceramic topcoat to the protective coating. Together, the protective coating (also referred to as a bond coat), the TGO layer, and the ceramic topcoat provide a Thermal Barrier Coating System (TBC) to protect the coated article.
Notwithstanding the protection provided by the thermal barrier coating system, the spallation and cracking of the thickening TGO scale layer often is the ultimate failure mechanism of conventional thermal barrier systems. Thus, improving the adhesion and integrity of the interfacial TGO scale is critical to the development of more reliable thermal barrier systems. Ideally, when exposed to high temperatures, the aluminum-containing protective coating should oxidize to form a slow-growing, non- or less “rumpling,” nonporous TGO layer that adhere well to the protective coating and the ceramic topcoat.
A conventional bond coat is typically either an MCrAIY overlay (where M is Ni, Co, Fe, or combination of them) or a diffusion aluminide coating. An MCrAIY overlay is generally applied by Electron Beam Physical Vapor Deposition (EB-PVD), High Velocity Oxy-Fuel (HVOF), Low Pressure Plasma Spray (LPPS) or Vacuum Plasma Spray (VPS). Diffusion aluminide coatings are generally formed by chemical vapor deposition (CVD), slurry coating, or by a diffusion process such as pack cementation, above-pack, or vapor (gas) phase deposition. Diffusion aluminide coatings have particularly found widespread use as protective coatings for superalloy components of gas turbine engines due to: (1) the diffusion process is not a line-of-sight process allowing components with complex geometry or with internal surfaces to be coated; and (2) the diffusion process is generally cost-effective as compared with overlay process.
Reactive elements have been incorporated into aluminide coatings to improve the oxidation protection provided by these coatings. Examples of reactive elements that have been proposed for aluminide compositions include Hf, Zr, Y, La and/or Ce. With small additions of reactive elements to aluminide coatings, the adherence of the protective oxide scale to the coatings/alloys and the oxidation resistance of the coatings/alloys at high temperature under aggressive atmospheres can be improved.
The significant improvement of oxidation resistance due to Hf addition has been indicated in many cases, such as Hf addition to β-NiAl cast alloys (Pint et al., “Effect of quaternary additions on the oxidation behavior of Hf-doped NiAl,” Oxidation of Metals, Vol. 59, Pages 257-283, 2003), and Hf addition to MCrAlY coatings materials (Giggins Jr., et al., U.S. Pat. No. 3,993,454, and Gupta et al., U.S. Pat. No. 4,585,481). In addition, Hf addition has also been proved that hafnium decreases the propensity for rumpling when it diffuses into the coating and to the growing aluminum oxide increasing their creep resistance (Tolpygo et al., “Effect of Hf, Y and C in the underlying superalloy on the rumpling of diffusion aluminide coatings,” Acta Materialia 56, Pages 489-499, 2008) as well as decreases the voids formed in the coating alloy at the metal-oxide interface during oxidation (Provenzano et al., “Void formation and suppression during high temperature oxidation of MCrAlY-type coatings,” Surface and Coatings Technology, 36, Pages 61-74, 1988).
Recently, U.S. Pat. No. 7,273,662 to Gleeson et al. taught two-phase γ-Ni+γ′ Ni3Al alloy compositions with a purpose of reducing the progressive roughening or “rumpling” of the bond coat surface during thermal exposure. Due to the significant beneficial effect of Hf addition, diffusion coating processes to incorporate Hf into diffusion aluminide coatings have been studied for several decades. As early as the 1970s, U.S. Pat. Nos. 3,951,642 and 3,996,021 to Chang et al. disclosed a pack cementation process to produce Hf modified aluminide coatings. Later, U.S. Pat. Nos. 5,989,733, 6,136,451, 6,689,422 to Warnes et al. and U.S. Pat. No. 6,602,356 to Nagaraj et al. disclosed a CVD process to produce Platinum (Pt)-aluminide coatings with or without Hf addition. U.S. Pat. Nos. 6,514,629 and 6,582,772 to Rigney et al. taught Hf—Si-modified Pt-aluminide coatings formed by the steps of providing a substrate, depositing layers containing the platinum, aluminum, hafnium, and silicon, and heating the layers so that the aluminum, hafnium, and silicon diffuse into the layer of platinum to form a protective layer.
Pack cementation, CVD and vapor phase process are three potential industrial diffusion coating processes for forming Hf-modified aluminide diffusion coatings. Among them, the vapor phase process has the potential to offer many advantages. The others have drawbacks. Though used with some successes, pack cementation processes for both hafnium-modified aluminide and simple aluminide coatings share the same disadvantages, such as the need for an inert filler, the obstruction of cooling holes, and the embedded particles on the formed coating surface. While avoiding these shortcomings, a significant disadvantage of using a CVD process to form a hafnium-modified aluminide coating is the considerable equipment cost. In view of these disadvantages of pack and CVD processes, alternative deposition methods, such as vapor phase process, have been sought.
The information of vapor phase process for Hf-modified aluminide coatings is limited. U.S. Pat. No. 6,332,931 to Das et al. disclosed a vapor phase coating process to produce aluminide-hafnide coatings by using Hf metal or Hf-containing metallic alloys as Hf donor materials; as a result, the synthesized coatings contain about 0.5 to about 60 weight percent hafnium and about 12 to about 38 weight percent aluminum. However, too high Hf content in aluminide coatings can lead to the formation of Hf-rich precipitated phases in the coating or on the coating surface, such as HfC, HfO2, and Ni2AlHf etc, and furthermore, deteriorate the mechanical properties and the oxidation resistance of the aluminide coatings. Therefore, there is a need to develop low Hf content aluminide coatings whose application on turbine engine components, as either environmental coatings or the bond coat in a thermal barrier coating system, can significantly improve the gas-turbine performance. Due to the difficulty on the control over Hf and Al codeposition and the complex process parameters of the diffusion coating process, even though Hf modified aluminide coatings have been investigated for several decades, very limited information of Hf-modified aluminide diffusion coatings for industrial applications has been reported.
A significant obstacle to the use of vapor phase processes has been the inability to adequately control the co-deposition of aluminum and reactive elements, such as Hf, from suitable sources to the article to be coated. Unfortunately, the resultant aluminide coatings tend to incorporate too much hafnium when vapor phase processes are used. This leads to coatings whose mechanical properties and/or service life can be unduly compromised. Therefore, there is a strong desire and need in the industry to be able to use vapor phase coating techniques in a way that offers improved control over the co-transfer of aluminum and reactive element(s) such as Hf.